Convective Addiction

Mid-February Thaw: Temporal Pattern Change on the Horizon

Posted on 10. Feb, 2011 by Jesse in Uncategorized

A large part of the CONUS has been under the influence of rather extreme winter weather conditions since December.   For example, the Groundhog Day blizzard set or tied 24-hour snowfall records in parts of OK, MO and IL.  On a more recent note, heavier snow banding produced another bout of 12″ – 20″ + of snow across parts of N OK and NW AR, with preliminarily record low temperatures being set overnight/this morning (02/10/11) in locales such as Bartlesville, OK (-28 degrees F) and -31 degrees F in Nowata, OK.  Also, the mid-Atlantic and New England region has been pounded with its fair share of significant winter storms this season.  There is some good news coming down the jet stream.  While long-range forecasts are muddled with uncertainty, all of the numerical models are indicating that least a temporal bout of more mild weather is in store for the next 7-10 days.  However, lest anyone get ambitious for an early Spring arrival, it should be noted that there is still five weeks left before the vernal equinox.  Therefore, this mid-February thaw shouldn’t be viewed as a permanent warming trend.

First we’ll take a look at two teleconnection patterns and their relationship to the overall synoptic environment across the CONUS.  The Pacific/North American teleconnection (PNA) is one mode of climate variability.  A positive (+) phase tends to feature a stronger East Asian upper-level jet stream pattern with a more easterly shift in the exit region of the upper-level jet towards the west coast of the U.S.  If the PNA becomes negative (-), then that East Asian jet pattern tends to retract back into the W Pacific, forming a blocking pattern over the upper N Pacific region.  This would often mean a troughing pattern sets up over the western U.S. coastal areas, with a ridge of high pressure forming over the eastern seaboard, trapping more of the arctic air poleward towards the high latitudes.  Under a more positive (+) PNA pattern, a ridge of high pressure anchors over the eastern Pacific/western U.S. coastal areas, while a troughing pattern sets over the eastern seaboard. This tends to be associated with more arctic high pressure systems setting up on the Plains, bringing frigid temperatures into the region.

Another teleconnection pattern to be examined is the North Atlantic Oscillation (NAO).  Recent observations have shown there to be a strong correlation between its negative phase effects and its positive phase effects on the overall synoptic pattern.  Much of this winter has seen a negative NAO index, thus resulting in the pattern that the CONUS has been stuck in for much of the season.  That yielded quite a few split flow scenarios, where we saw more of a troughing pattern in the east, favoring a storm track that blasted the eastern seaboard with magnanimous record producing snowstorms.  For much of the rest of the country, it meant below normal temperatures and unseasonably dry conditions across parts of the Plains.

On the contrary, when the NAO enters a more neutral into a positive phase, the opposite tends to occur, with an overall environment that begins to become more conducive to springtime severe weather events.  When a trough anchors over the western CONUS, it tends to allow the promulgation of milder air to return poleward (northward), lending a more favorable thermodynamic environment to coalesce  with cyclones that traverse the more SW-NE oriented pattern.

Found below is the NAO chart showing past and future (forecast) NAO gyrations, which shows what we’ve been dealing with all winter, while also highlighting what’s expected into the end of meteorological winter.  From the end of November into December, the NAO entered a highly negative phase, resulting in epic winter storms across the northeast and major cold spells across much of the central region.  As we entered February, we noticed a change in the overall pattern, with the most recent Groundhog Day blizzard being indicative of this change.  With the NAO entering a more neutral/positive phase, we can, at least for a time, expect a return to milder temperatures and more SW – NE storm track, and this suggested pattern change is being supported by the numerical models as well.

First, let’s take a look at both Thursday’s 12z GFS forecast of surface temperatures and pressure systems as well as the ECMWF (European model) forecast surface pressure over the next 7-8 days.

Now let’s take a look at the 500 mb flow over the next ten days as the pattern modifies itself from an east coast trough to a west coast trough. Below we can see the GFS showing a favorable troughing pattern.

The animations below will do this more justice, but note the still shot of the upcoming pattern change.

So while it is still early February and we undoubtedly have more winter to contend with, we are beginning to see signs that a more spring-like pattern may be on the horizon.  It seems almost cliche to think that mother nature follows the calendar, but after a long and harsh winter, seeing these signs again on long-range forecasts is certainly promising.  At this point in time it is pointless getting excited over any individual setup being shown on the long-range pattern.  What is more important is to note what is being shown in the general pattern, and right now it is certainly pointing towards a pattern that *could* lend an early season chase setup or two.  If nothing else, a brief respite from the arctic blasts is in store.

As always, we will be on top of the ever-changing situation and will post more updates as needed.

Synopsis written by Convective Addiction’s Adam Lucio and Jesse L. Risley